MongoDB Query to Unwind Array with $unwind Operator

Use the MongoDB aggregation pipeline stage { $unwind: "$arrayField" } to flatten an array field into multiple documents, one per array element.

Examples

Input{ _id: 1, fruits: ["apple", "banana"] }

Output{ _id: 1, fruits: "apple" } { _id: 1, fruits: "banana" }

Input{ _id: 2, colors: ["red"] }

Output{ _id: 2, colors: "red" }

Input{ _id: 3, items: [] }

Output

How to Think About It

To unwind an array in MongoDB, think of splitting each document into multiple documents, one for each element in the array. The $unwind operator does this by taking the array field and creating a separate document for each item inside it.

Algorithm

1

Start with the collection containing documents with array fields.

2

Apply the <code>$unwind</code> stage on the array field you want to flatten.

3

MongoDB outputs one document per element in the array, duplicating other fields.

4

Collect the resulting documents as the output of the aggregation.

Code

mongodb

db.collection.aggregate([
  { $unwind: "$fruits" }
])

Output

{ "_id" : 1, "fruits" : "apple" }
{ "_id" : 1, "fruits" : "banana" }

Dry Run

Let's trace unwinding the array ["apple", "banana"] in the fruits field.

1

Original Document

{ _id: 1, fruits: ["apple", "banana"] }

2

Apply $unwind

Split into two documents: { _id: 1, fruits: "apple" } { _id: 1, fruits: "banana" }

_id	fruits
1	apple
1	banana

Why This Works

Step 1: Why use $unwind?

The $unwind operator breaks down arrays so you can work with each element individually.

Step 2: How it works

It creates a new document for each element in the array, copying other fields as is.

Step 3: Result

You get multiple documents from one, each with a single array element.

Alternative Approaches

Use $unwind with preserveNullAndEmptyArrays

mongodb

db.collection.aggregate([
  { $unwind: { path: "$fruits", preserveNullAndEmptyArrays: true } }
])

Keeps documents with empty or missing arrays instead of removing them.

Use $project with $arrayElemAt for specific elements

mongodb

db.collection.aggregate([
  { $project: { firstFruit: { $arrayElemAt: ["$fruits", 0] } } }
])

Extracts only one element instead of unwinding all; useful for limited cases.

Complexity: O(n*m) time, O(n*m) space

Time Complexity

The operation processes each document and each element in the array, so time grows with total array elements (n documents * m elements).

Space Complexity

Output size grows with the number of array elements, as each element produces a new document.

Which Approach is Fastest?

Using $unwind is efficient for flattening arrays; alternatives like $project extract specific elements but don't flatten.

Approach	Time	Space	Best For
$unwind	O(n*m)	O(n*m)	Flattening arrays fully
$unwind with preserveNullAndEmptyArrays	O(n*m)	O(n*m)	Flattening while keeping empty arrays
$project with $arrayElemAt	O(n)	O(n)	Extracting specific array elements